1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a lateral double-diffused metal-oxide-semiconductor (LDMOS) having a uniform distribution of channel impurity concentration.
2. Discussion of the Related Art
Referring to FIG. 1, illustrating a conventional LDMOS transistor, an n− semiconductor substrate 100 has active region set according to a device isolation layer 110. A p-type body 120 and an n− extended drain region 130 are formed in the n− semiconductor substrate 100 to be separated from each other by a predetermined distance. An n+ source region 140 is disposed on the p-type body 120. A channel 121, occurring in the p-type body 120 adjacent the n+ source region 140, is overlapped by a gate isolating layer 160 and a gate conducting layer 170, which are sequentially formed atop the channel. Spacers are formed on the sidewalls of the gate conducting layer 170. An n+ drain region 150 is disposed on the n− extended drain region 130. The structure is completed by a double diffusion process in which an ion implantation process is carried out twice, i.e., once before formation of the gate spacer layer 180 and again after its formation. The source and drain regions 140 and 150 are electrically connected with a source electrode S and a drain electrode D, respectively.
FIG. 2 shows the relative impurity concentrations with respect to the channel of the LDMOS transistor of FIG. 1. For example, the n+ source region 140 exhibits an impurity concentration curve A1, which is relatively high and decreases toward the channel 121; the channel exhibits an impurity concentration curve B1 that is relatively low and decreases toward the n− drift region of the semiconductor substrate 100; and the drift region exhibits a uniform impurity concentration curve C1. To obtain a desirable breakdown voltage, the channel 121 should show a uniform distribution of its ion concentration. Since a contemporary LDMOS transistor has a grade junction, however, the length of the channel 121 cannot be reduced, which limits device integration.